The present invention generally relates to circuits that determine the presence of a load in a circuit. In particular, the invention relates to circuits that determine the presence of a load in a communications link.
Modem portable electronic devices, including laptop computers, personal digital assistants, and cellular telephones, are generally powered by rechargeable batteries. The operating time derived available from a fully charged battery is limited. Once the battery can no longer provide a minimum voltage or power, it must be recharged before subsequent use or replaced with a charged battery. Consequently, it is desirable to reduce power consumption during times when the device is not fully operational to extend battery life. By selectively deactivating device features that are not required, power consumption can be substantially reduced. Unfortunately, it is generally not practical for a device user to determine when a device should be operated in a reduced power mode. For example, when communicating with a remote device, the user may not recognize when communications have been interrupted or terminated, especially if the communications do not require interactions with the user. Thus it is desirable for the device to automatically determine when communications have ended. The present invention relates to circuits and method to determine these inactive periods.
In one aspect, the present invention relates to a circuit for determining the presence of a load connected to the circuit. In one embodiment, the circuit includes a charge pump and a monitor module. The charge pump has a pump output terminal providing a pump signal indicative of the operating state of the charge pump with a duty cycle indicative of the time the pump signal is in a first state. The monitor module has a monitor output terminal and a monitor input terminal in electrical communication with the pump output terminal. The monitor module provides a status signal indicative of the presence of a load connected to the circuit in response to the duty cycle of the pump signal. In one embodiment the monitor module includes a comparator with a first comparator input terminal in electrical communication with the monitor input terminal, a second comparator input terminal configured to receive a first reference voltage, and a comparator output terminal in electrical communication with the monitor output terminal.
In one embodiment, the monitor module also includes a filter module with a filter input terminal in electrical communication with the monitor input terminal and a filter output terminal in electrical communication with the first comparator input terminal. The filter module generates a voltage substantially proportional to the duty cycle of the pump signal. In another embodiment, the filter module includes a charge module, a discharge module, and a capacitive element. The charge module has a charge input terminal connected to the filter input terminal and a charge output terminal connected to the filter output terminal. The discharge module has a discharge input terminal connected to the filter input terminal and a discharge output terminal connected to the filter output terminal. The capacitive element has a first capacitive terminal connected to the filter output terminal and a second capacitive terminal configured to receive a third reference voltage.
In one embodiment, the charge module includes a PMOS transistor with a source in electrical communication with the filter input terminal, a drain in electrical communication with the first capacitive terminal, and a gate configured to receive a voltage indicative of the supply voltage. In another embodiment, the discharge module includes a NMOS transistor with a source connected to the filter input terminal, a drain connected to the first capacitive terminal, and a gate receiving a voltage indicative of the supply voltage. In yet another embodiment, the charge module includes multiple charge modules each having a charge input terminal electrically coupled to the filter input terminal and a charge output terminal electrically coupled to the first capacitive terminal. In yet another embodiment, each of the multiple charge modules is designed to operate at a particular voltage indicative of the supply voltage.
In another aspect, the present invention provides a method for determining the presence of a load in a circuit having a charge pump. In one embodiment, the method includes the steps of receiving a pump signal indicative of the operating state of the charge pump, determining the duty cycle of the pump signal, and determining in response to the duty cycle whether a load is present in the circuit. In another embodiment, the method also includes the step of generating a status signal indicative of the presence of the load in the circuit in response to the determination of the presence of the load in the circuit. In yet another embodiment, the step of determining whether a load is present in the circuit in response to the duty cycle includes the steps of filtering the pump signal to generate a filtered signal having a substantially DC voltage substantially proportional to a time average of the pump signal and comparing the filtered signal to a reference voltage.